Various WP8 fixes

Added BackButton support (with exit)

Modified template to support back button
Moved game creation in template after load to correctly set the size (if
created earlier, width and height are set to 0 resulting in infinite touch locations among others)

Don't start playing a SoundEffectInstance if it's playing (this was an issue on Windows 8 too)

Added support for music playback on WP8 
Fixed PlayPosition on Windows 8